His tactless criticism of his colleague's work during the meeting was very inappropriate.

tactless 是由名词 tact（机智、圆滑）加上否定后缀 -less（表示“没有”）构成的形容词。tact 指“在社交场合中得体、机智的言行方式”，-less 表示“缺乏”，所以 tactless 意思是“不得体的、不圆滑的”。